Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current. This p-n heterojunction is defined by the intermixing of the CdTe layer and the CdS layer at the junction, allowing this heterojunction to be characterized by the chemical formula CdTe1-xSx along the junction surface of the CdTe layer and CdS1-xTex along the junction surface of the CdS layer. As such, the heterojunction between the CdTe layer and the CdS layer can be described as a mixed combination of the layers.
This intermixing of the CdTe layer and the CdS layer at the junction can, however, lead to undesired diffusion of ions and/or dopants between the CdTe layer and the CdS layer that can decrease the efficiency and/or lifetime of the device. For example, copper dopants added to the CdTe layer can diffuse over time across the heterojunction and into the CdS layer, leading to reduced efficiency and/or lifetime of the device.
Thus, a need exists for cadmium telluride photovoltaic devices having improved energy conversion efficiency and/or device lifetime through reduced dopant and/or ion diffusion between the CdTe layer and the CdS layer and aiding in reducing recombination of generated carriers at the interface of the CdTe layer and the CdS layer.